严禁 (yán jìn) is a formal, written verb meaning 'to strictly prohibit' — it is stronger than 禁止 (jìn zhǐ) and carries a sense of absolute, no-exceptions ban, often used in rules and signs. 不许 (bù xǔ) and 不准 (bù zhǔn) are more colloquial and less forceful, while 严格 (yán gé) is an adjective meaning 'strict', not a prohibition verb.